Injector for a fuel injection system for internal combustion engines, with hydraulic prestressing of the pressure booster

ABSTRACT

An injector, actuated by a piezoelectric actuator with a hydraulic booster, for a fuel injection system for internal combustion engines is disclosed, in which the pressure booster is always hydraulically prestressed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 00/03582 filed onOct. 12, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an injector for a fuel injection system forinternal combustion engines, having a pressure booster that has a firstpiston guided in a first guide bore and a second piston guided in asecond guide bore, in which the first piston and the second pistondefine a pressure chamber that is under at least a supply pressure, andhaving a leaking oil line, the first piston being actuated by anactuator and the first piston at least indirectly actuating the finalcontrol element of a control valve, and to a fuel injection system forinternal combustion engines using the injector.

2. Description of the Prior Art

To make it possible to assure a defined operating state of the injectorand consequently good operating performance, and above all favorableengine starting performance, the second piston of the hydraulic pressurebooster has until now been pressed against the final control element ofthe control valve by a spring, such as a cup spring.

This arrangement assures that the second piston of the hydraulicpressure booster will always assume a defined position, but airinclusions or the like can have the effect that a defined operatingstate of the injector is still lacking, and in particular that theengine starting performance is inadequate.

OBJECTS AND SUMMARY OF THE INVENTION

The primary object of the invention is to furnish an injector for a fuelinjection system for internal combustion engines that has a pressurebooster with improved operating performance.

This object is attained according to the invention by an injector for afuel injection system for internal combustion engines, having a pressurebooster that has a first piston guided in a first guide bore and asecond piston guided in a second guide bore, wherein the first pistonand the second piston define a pressure chamber that is under at least asupply pressure, and having a leaking oil line, wherein the first pistonis actuated by an actuator and the first piston at least indirectlyactuates the final control element of a control valve, in which thesecond guide bore communicates hydraulically with the leaking oil returnand the pressure p₁ in the leaking oil return is lower than the supplypressure.

As a result of the prevailing pressure difference between the pressurechamber and the leaking oil return, a hydraulic force that presses thesecond piston against the final control element of the control valvealways acts on the second piston.

As a result, not only is a defined position of the second piston of thehydraulic pressure booster assured, but in addition, there is a constantexchange of the fuel, or possible air inclusions, in the pressurebooster.

Because of the low pressure level in the injector directly downstream ofthe control valve, the operating performance of the outlet throttledisposed between the control chamber and the control valve alsoimproves.

In variants of the invention, the first guide bore, the first piston andthe housing are sealed off from their surroundings, in particular by abellows, so that no leakage can occur, and the supply pressure in thepressure chamber remains high.

Further in the invention, it is provided that supply pressure in thepressure chamber is furnished by a supply line discharging into thechamber formed by the housing, bellows and first piston, so that all thelines (31, 25, 27) and chambers (34, 17, 29) always have a flow of fuelthrough them.

In a further feature of the invention, a pressure holding valve isdisposed in the supply line, so that pressure p₂ in the supply line islimited, in particular to 50 bar.

Another embodiment has a recess which is in communication with theleaking oil return at the second guide bore, so that the control andleakage quantities can be returned without problems.

In a further variant of the invention, the second piston actuates thefinal control element of the control valve via a tappet, so that theportions of the second guide bore that lead between the second pistonand the tappet need not be precisely aligned, and different tolerancesare possible.

Further in the invention, the outlet side of the control valvedischarges into the leaking oil return, so that the control quantity iscarried away.

In another feature of the invention, the actuator is a piezoelectricactuator, thus making major control forces available.

Another embodiment provides for mounting the housing, the control valve,and the actuator in a retaining body, thus on the one hand facilitatingmanufacture and mounting and on the other making a play-free,prestressed connection of the components possible.

The object stated above is also attained by a fuel injection system forinternal combustion engines, having a high-pressure fuel pump, having ahigh pressure fuel reservoir, and having at least one injector accordingto the invention, so that the advantages of the invention come into playin this fuel injection system as well.

BRIEF DESCRIPTION OF THE DRAWING

Further objects and advantages of the invention can be learned from theensuing description, taken in conjunction with the single figure of thedrawing which is a sectional view schematically showing an injectorembodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawing, one exemplary embodiment of an injector of theinvention, with a hydraulic pressure booster, is shown. In a two-parthousing 1 and 3, a first guide bore 5 and a second guide bore 7 aredisposed coaxially to one another. A first piston 9 is guided in thefirst guide bore 5. The second guide bore 7 guides a second piston 11and a tappet 13. Via the tappet 13, the second piston 11 acts on thespherical final control element 15 of the control valve, not shown, ofthe injector. Because of the two-piece design of the second piston 11and tappet 13, the two portions of the second guide bore 7 need not bealigned precisely with one another; furthermore, the play between thesecond piston 11 and the second guide bore 7 and between the tappet 13and the second guide bore 7 can be selected to be different.

The first piston 9 and the second piston 11 define a pressure chamber 17of a hydraulic pressure booster 19. A piezoelectric actuator 21 acts onthe first piston 9. When the piezoelectric actuator 21 is actuated, itpresses the first piston 9 in the direction of the arrow into thepressure chamber 17. As soon as the consequently rising pressure in thepressure chamber 17 suffices to exert a hydraulic force on the end faceof the second piston 11 protruding into the pressure chamber 17 that isgreater than the hydraulic force exerted by the fuel under controlpressure in the control chamber 23 on the final control element 15, thefinal control element 15 opens and establishes a hydraulic communicationbetween the control chamber 23 and a leaking oil return 25. Thiscommunication is effected via a portion of the second guide bore 7, asupply line 27 and a recess 29 of the second guide bore 7. As a result,the pressure in the control chamber 23 drops, and tie injection nozzle,not shown, opens.

As soon as the actuation of the piezoelectric actuator 21 ends, thepiezoelectric actuator moves, together with the first piston 9, backinto its outset position counter to the direction of the arrow. As aresult, the pressure in the pressure chamber 17 drops. A supply line 31,in which a supply pressure p₂ prevails, supplies the pressure chamber 17with fuel indirectly, via a bore in the housing 1 and a bellows 33.

The housing 1 and the first piston 9 are sealed off from theirsurroundings by the bellows 33. The supply line 31 discharges into thechamber 34 defined by the housing 1, first piston 9 and bellows 33 andfills this chamber with the fuel, which is at the pressure p₂. Throughthe annular gap between the first guide bore 5 and the first piston 9,fuel reaches the pressure chamber 17, so that this pressure chamber isalways filled with fuel, and also, at least approximately, a pressure inthe amount p₂ always prevails in the pressure chamber 17.

In the recess 29 as in the leaking oil return 25, a pressure p₁prevails, which is lower than the supply pressure p₂. Consequently, ahydraulic force oriented from the piezoelectric actuator 21 to the finalcontrol element 15 acts on the second piston 11. As a consequence ofthis exertion of force, the second piston 11 and the final controltappet 13 are pressed against the final control elements 15, so that noplay can arise between the second piston 11, tappet 13 and final controlelement 15. Instead, the pressure booster 19 is prestressed.

Since only the pressure pi prevails in the recess 29, fuel flows out ofthe pressure chamber 17 into the recess 29 through the annular gapbetween the second guide bore 7 and the second piston 11. Besides theaforementioned effect that the pressure difference between the pressurechamber 17 and the recess 29 connects the piston to the final controlelement 15 without play, the described flow of the fuel from the supplyline 31 as far as the leaking oil return 25 has the effect that theentire pressure booster always experiences a flow of fresh, relativelycool fuel through it, thus lessening the risk of seizing; furthermore,any air or vapor bubbles that may be present are always conveyed out ofthe pressure booster 19 into the leaking oil return 25.

Another advantage of the embodiment according to the invention is thatin contrast to the prior art, the bellows is always subjected topressure only statically. As a result, an indication of the supplypressure p₂ without having to modify the bellows 33 is possible, andthus also an increase in the prestressing of the piezoelectric actuator.The supply pressure p₂ can for instance amount to 50 bar.

The entire injector, comprising the piezoelectric actuator 21, pressurebooster 19 and the control valve, not shown, and the actual injectionnozzle itself, are held together by a retaining body 35. The fuelflowing out of the supply line 31 through the retaining body 35 isprevented from flowing out into the surroundings by two O-rings, whichare disposed between the retaining body 35 and the housing 1.

In the supply line 31, there is a pressure holding valve 37, whichassures that an allowable maximum pressure in the supply line 31 and inthe chambers 17 of the pressure booster that communicate hydraulicallywith the supply line 31 is never exceeded. As soon as this maximumpressure is reached, the pressure holding valve 37 opens. The outletside of the pressure holding valve can be in communication with theleaking oil return 25, for instance.

The foregoing relates to preferred exemplary embodiment of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

I claim:
 1. An injector for a fuel injection system for internalcombustion engines, comprising a pressure booster (19) having a firstpiston (9) guided in a first guide bore (5) and a second piston (11)guided in a second guide bore (7), wherein the first piston (9) and thesecond piston (11) define a pressure chamber (17) that is under at leasta supply pressure, and having a leaking oil return line (25), whereinthe first piston (9) is actuated by an actuator (21) and the secondpiston (11) at least indirectly actuates the final control element (15)of a control valve, said second guide bore (7) communicateshydraulically with said leaking oil return (25), the pressure p₁ in theleaking oil return (25) being lower than the supply pressure.
 2. Theinjector of claim 1, wherein the first guide bore (5), the first piston(9) and the housing (1, 3) are sealed off from their surroundings. 3.The injector of claim 2, further comprising a bellows (33) sealing offsaid first guide bore (5), said first piston (9) and said housing (1, 3)from their surroundings.
 4. The injector of claim 2, wherein the supplypressure in the pressure chamber (17) is furnished by a supply line (31)discharging into the chamber (34) formed by the housing (1, 3), bellows(33) and first piston.
 5. The injector of claim 1, further comprising apressure holding valve (37) disposed in the supply line (31).
 6. Theinjector of claim 5, wherein said pressure holding valve (37) limits thepressure p₂ in the supply line (31) to about 50 bar.
 7. The injector ofclaim 1, further comprising a recess (29) which is in communication withthe leaking oil return (25) is present at the second guide bore (7). 8.The injector of claim 1, wherein said second piston (11) actuates thefinal control element (15) of the control valve via a tappet (13). 9.The injector of claim 1, wherein the control valve discharges into theleaking oil return (25).
 10. The injector of claim 1, wherein theactuator is a piezoelectric actuator (21).
 11. The injector of claim 1,wherein the housing (1, 3), the control valve, and the actuator (21) aremounted in a retaining body (35).
 12. A fuel injection system forinternal combustion engines, including a high-pressure fuel pump, a highpressure fuel reservoir, and at least one injector said injectorcomprising a pressure booster (19) having a first piston (9) guided in afirst guide bore (5) and a second piston (11) guided in a second guidebore (7), wherein the first piston (9) and the second piston (11) definea pressure chamber (17) that is under at least a supply pressure, andhaving a leaking oil return line (25), wherein the first piston (9) isactuated by an actuator (21) and the second piston (11) at leastindirectly actuates the final control element (15) of a control valve,said second guide bore (7) communicates hydraulically with said leakingoil return (25), the pressure p₁ in the leaking oil return (25) beinglower than the supply pressure.
 13. The injector of claim 3, wherein thesupply pressure in the pressure chamber (17) is furnished by a supplyline (31) discharging into the chamber (34) formed by the housing (1,3), bellows (33) and first piston.
 14. The injector of claim 4, furthercomprising a pressure holding valve (37) disposed in the supply line(31).
 15. The injector of claim 13, further comprising a pressureholding valve (37) disposed in the supply line (31).
 16. The injector ofclaim 13, wherein said pressure holding valve (37) limits the pressurep₂ in the supply line (31) to about 50 bar.
 17. The injector of claim 2,further comprising a recess (29) which is in communication with theleaking oil return (25) is present at the second guide bore (7).
 18. Theinjector of claim 2, wherein said second piston (11) actuates the finalcontrol element (15) of the control valve via a tappet (13).
 19. Theinjector of claim 2, wherein the control valve discharges into theleaking oil return (25).
 20. The injector of claim 4, wherein theactuator is a piezoelectric actuator (21).
 21. The injector of claim 4,wherein the housing (1, 3), the control valve, and the actuator (21) aremounted in a retaining body (35).